High end bikes are generally lighter, yes. It has not been my experience that higher end shimano component groups shift better. I have tiagra 10. I also test rode 105 10, 105 11 and ultegra 11 multiple times. I did not notice a difference in shift quality among these groups.

I test rode high end spesh roubaix bikes but didn't feel like the ride was especially plush. Maybe a hair more "comfortable" than alu.

I don't think you get more "attention to detail." You get more carbon, which is lighter. But you also have to be more careful with torque, and these components can shatter rather than bend.

Several of the carbon bikes I test rode also had odd creaks and noises which was very annoying.

Carbon wheels are LOUD. And they brake poorly.

The industry gives you a fractionally lighter bike. But for me, it's not worth it. I don't want noisy, poor braking rims. I don't want to have to use a torque wrench every time I adjust a seatpost or saddle, or install pedals on carbon crank arms.

Plus, if I want to save some weight, I can buy a pair of lightweight wheels, and save a pound and a half off the bike for only $350 or less.

Take it the other way: if your bike is heavier by a few pounds, it will be a superior training bike, since you will have to work a little bit harder. But training for a race is really not an objective for 99.99999% of bicyclists.

Even for those who purchase $6K+ bikes, the majority will never enter a sanctioned race.